Recovery of uranium from leached mud of the clinker process for producing phosphoric acid



United States Patented Sept. 17, 1957? 2,866,763 RECOVERY OF URANIUM FROM LEACHED MUD @F THE QLENKER PRUCESS F812 PRODUCING PHDSPHURIC Atlm Frederick T. Pitch and Mary P. From, Baltimore, Md, assignors to W. R. Grace & C0,, Norwalk, Comm, a corporation of Connecticut No Drawing. Application February 2, H52, Serial No. 269,685 (Ilaims. (Cl. 23-145) This invention relates to the recovery of uranium from phosphate rock. In a morespecific aspect, it relates to the recovery of uranium from the leached residue of the so-called Clinker process for the manufacture of phosphoric acid. In another specific aspect, it relates to the recovery of uranium from phosphate rock from which the phosphate has been leached under such conditions that the uranium is left as an insoluble constituent of the leached mud.

In recent years, the importance of uranium as a source of atomic power has greatly accelerated the search for domestic sources of this valuable material. In view of the shortsupply of uranium, very low grade ores must be carefully considered as sources. As a consequence of the great need and the shortage of important sources, commercial grade phosphate rock, which contains about 0.01 to 0.02% uranium is coming under consideration as a possible source. Large amounts of this material are worked annually in the manufacture of phosphoric acid for making fertilizers so that the total amount of uranium is considerable if a method can be developed for recovering it.

At least one such method is receiving attention. This involves acidulation of the phosphate rock in such a manner that the uranium is solubilized and must be recovered subsequently from the phosphoric acid: product. In co-pending application Serial No. 269,684, filed February 2, 1952, now abandoned, there is described a method of treating phosphate rock to solubilize the phosphate and insolubilize the uranium so that most of the uranium remains in the insoluble residue after leaching of the phosphoric acid. Thus, the leached mud contains substantially all of the uranium. It is now desirable that there be developed a process for recovering the uranium from this leached mud. Our efforts were directed to this end with the result that the method of the present invention was developed. In the process described in the above mentioned co-pending application, phosphate rock is acidulated with sulphuric acid under substantially anhydrous conditions and the soluble phosphate is removed by Water leaching. This leaves a calcium sulfate residue containing substantially all of the uranium from the original rock but which is substantially free of phosphate and fluorine.

' In carrying out the present invention, the leached material is drained and dried. The conditions of drying are determined by convenience since the drying temperature is not critical. The dried material is then reduced to a convenient particle size for subsequent acid leaching. Our work has shown that the particle size has little effect on the operation. In this connection, a thru 12 mesh material has been satisfactorily handled. The dried calcium sulfate residue or mud is then roasted or calcined at a moderately high temperature-generally above about 1250 F. This minimum roasting temperature may vary somewhat between different muds. Below the minimum temperature for a particular mud, the percent recovery of uranium is low. Above this minimum, recovery of uranium increases rapidly with increasing roasting temperature until an optimum temperature is reached (about 110 to 130 degrees above the minimum).

Above the optimum, the elfect of further temperature increase on uranium recovery becomes less pronounced although it may still be slightly beneficial. In practice, we prefer to roast the dried mud at a temperature between the optimum and about 150 degrees thereabove.

This gives a good operating range and the beneficial eifect of higher temperatures is not sufficient to warrant their use.

Each mud, according to its source and prior treatment has a characteristic optimum roasting temperature which can be easily determined. In addition, the conditions of roasting also affect the optimum temperature. For example, the optimum temperature for a given mud will be slightly lower if the roasting is carried out under quiescent conditions than if the material is agitated during roasting. It is preferred to agitate the material during roasting despite the somewhat higher optimum temperature because the recovery of uranium is higher under these conditions. In addition, stationary bed heating is harder to control because of irregular heat transfer. which may reuslt in somewhat irregular recoveries.

For agitated bed roasting, the optimum temperature for a leached residue from the Clinker process using a 67-73% BPL Bonny Lake phosphate rock is between about 1360 F. andabout 1475" F. For quiescent roasting, the preferred range is between about 1250 F. and 136091 above which, the results may be somewhat irregular.

Our tests have shown that a roasting period of about 15 minutes within the proper range is generally sufficient and the additional beneficial effect of prolonged heating is only slight.

Generally, a rotary calciner is a preferred type of apparatus for this operation although other equivalent calcining devices may be used.

The roasted material is treated with a dilute solution of a strong mineral acid, for example, sulfuric, nitric, hydrochloric, and phosphoric, to solubilize the uranium. This may be done by pouring the roast, either hot or cold into a sufiicient volume of the dilute acid to wet the roast and allowing it to soak for a short time, during which it is, preferably, stirred. The volume of acid should be sufiicient to permit adaquate working of the mixture. Less than about /3 by weight of the dry roast would probably be a lower limit with the upper limit determined by economic considerations. Between 50 and weight percent is an efiicient range.

The amount of acid must be in excess of that necessary to dissolve the small quantities of acid-soluble constitu-' ents, which includes small amounts of iron and aluminum mum recovery. However, this is due to the poorly prepared roast.

The acid leach solution may be hot or cold although the former gives slightly better results. The acid-roast mixture should be stirred just enough to insure adequate contact between the solid and liquid. Constant or vigorous stirring may have a detrimental eifect on the subsequent filtration. While a major portion of the recovery is to be obtained after about /2 to 1 hour of acid leaching, maximum recovery requires about 2 hours of leaching. After about 2' hours the recovery'beginstm decrease with increased 1 sulfuric acid, becauserof its-ready. availability, low cost,

and ease. of handlingisitor be preferred;

After. the. acid'digestion, the mud; is filtered and" washed withv water using. well-knownteichniques: to'- result. in most eflicient washing. Thus, the acidisolution may be first drained from the roast mud, which is then washed av number of times with. cold watett. It. is desirable. that the WashLWateIr notibeusett imexcess; because. the recovery of the: uranium frorrr excessively. large. volumes of water may be. unnecessarily. complicated. About 70 to. 80% offtheuraniuml originally presentiin theroasted mud will be easily 'recoverectintheiacii and.washings; It ispresent in. solution. as the uranyl ion and its removal. and. subsequent recovery can. be: carried out. by known methods. The solution alsomay contain: small. amounts of. alumina,

. iron, or other metal sa'lts'whichcan, likewise, be separated by conventional procedures.

For purposes of:convenience; the; term Clinker process is used inthe specification. and claims to refer to that:

process for producing phosphoric acid wherein. ground phosphate rock is'acidulatecLwith substantially anhydrous sulfuric acid to convert all of the phosphate to phosphoric acid after which the acidulated. material is calcined at anelevated temperature to produce a relatively dry solid from'which the phosphoric acid. is leached". with water.

This relatively dry solid is referred to in the industry as.

V a Clinker, which accounts. for the. name;

"The present invention is: further illustrated by the fol.- lowing non-limiting examples.

Example I Samples of leached: calcium sulfate residue from Bonny Lake phosphate rock Which;.hadr been acidulated' with- 98% sulfuric acid and'calcined as described in the above co-pending application prior to. leaching were dried and ground as described herein. The ground samples were roasted at various temperatures for: uniform periods of 15 minutes. Each roa'st'wasthen soaked for two hours in an equal weight of 114%- sulfuric acid, after which it was drained and washed.v The combined acid-and washing's from each sample were. then: tested for the uranium content. The percentrecovery based onthe total amount Exam le 11 7 Two portions of the material. designated as sample. A in Example I were roasted at the same temperature but for different periods. and: were extracted. asdescribed in Example I. The results. were. as follows.

Sample Bed Condition .Temp, Time, Percent 1 F. mln.- Recovery a "agitated r.. 1,365 V 15 73' k dn' i 1 1,365 30 77 41 Example III Tests were made with various roasts, prepared as described in Example I, to determine the effect of leach acid concentration on the uranium recovery. A 1 to 1 acid to roast ratio was used. The following results were obtained.

Roast H2804 Condl- Time, Concen- Percent Semple Bed tions, min. tratton, Recovery Temp, percent.

A Agitated. 1, 365 15 2 67 A.-- ..do 1,365 15 4 71 A..- do 1,365 15 8 71 .do; 1, 365 15 14 73 Stationary l, 250 15 3. 6 15 0 1, 250 15 14 48 1, 425 50 2- 1,425 15 4. 6n 1, 425' 15 8' 71 1, .425 15 14 70- Example' IV Axlarge batch offthe leached calcium sulfate. residue, such. as that: describedinExample. I.was drained and dried at about 250 F. and was then ground to pass an 8'mesh screen. While. the original. raw rock contained 0.014 weight percentof uranium. as'UaOs, the dried phosphatefree material contained 0.01%..

Thisbatch-v of. residue was then heated in a direct' fir rotary calciner'having a minimum retention time of about 5.0. minutes. and a. maximum. bed temperature of about mud of the Clinker process. for producing phosphoric: 7

acid comprising drying said mud, grinding the dried material', roasting saidground and. dried material at" above: about 1245 F-., digesting the roasted material with a dilute solution of a strong mineral acid, and washing. the acid andsolubilized uranium therefrom-with water.

2. The method of recovering'uranium from the leached. mud of the Clinker process for producing phosphoric acid comprising drying said leached mud, grinding the dried.

material to an average particle size of about 810. mesh, roasting said groundandidried material at a temperature above about 1245 R, digesting the roasted material with an equal weight of a dilute solution of a strong mineral acid, and washing the acid and solubilized uranium there.-

from with water. 7

3. The method of recovering uranium'from the leached mud of the Clinker process for producing phosphoric acid comprising drying said leached mud, grinding the. dried material to a particle size of about 10 mesh, roasting said.

dried material at a temperature-between 1360-1.475 F. for at least 15' minutes, digesting the roasted material with an equal weight of a 2%-14% solution of a; strong.

mineral acid, and washingthe acid and solubilized urae nium therefrom with water.

4. The method of recovering uranium from the leached mud of the Clinker processfor producing phosphoriciacidi. comprising grinding the dried-material, roasting saidi dried? material at a temperature above about 1245 for at least 15 minutes, digesting the roasted material with an:

equal weight of about 2%14% sulfuric acid and washing.

the acid and solubilized uranium therefrom with-water" 5i The-method of. recovering uranium from the leached".

mud of the Clinker process for producing phosphoric acid comprising drying said leached mud, grinding the dried material to a particle size of about mesh, roasting said dried material at a temperature of 1360 F. to l475 F. for at least minutes, digesting the roasted material with an equal weight of 14% sulfuric acid for a period of 1-2 hours, and washing the acid and solubilized uranium therefrom with water.

6. The method of recovering uranium from the leached mud of the Clinker process for producing phosphoric acid comprising drying said leached mud, grinding the dried material to a particle size of about 8-10 mesh, roasting said dried material at a temperature between 1360" F. and 1475 F. for about 15 minutes, digesting the roasted material with an equal weight of a 2%-l4% solution of a strong mineral acid for a period of less than 2 hours, and washing the acid and solubilized uranium therefrom with water.

7. The method of recovering uranium from the leached mud of the Clinker process for producing phosphoric acid comprising drying and grinding said mud, roasting said dried material at a temperature of 1360 F. to 1475 F., digesting the roasted material with a 2%-14% solution of a strong mineral acid selected from the group consisting of sulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid, and washing the acid and solubilized uranium from said roast with water.

8. The method described in claim 12 wherein the strong mineral acid is sulfuric acid.

9. The process of recovering uranium from the leached mud of the Clinker process for producing phosphoric acid comprising drying said mud, grinding the dried material to a medium particle size, roasting the ground material at a temperature above about 1245 F. for a period of at least 15 minutes, leaching the roasted material with a 2%14% aqueous solution of a strong mineral acid for a period of less than about 5 hours, and washing said mud to remove acid and uranium therefrom.

10. The method of recovering uranium from the leached mud of the Clinker process for producing phosphoric acid comprising the steps of drying said mud, grinding the dried material to an average particle size of about 8-10 mesh, roasting the ground material for at least 15 minutes at a temperature of about 1360 F. to 1475 F., digesting said roasted material with a dilute aqueous solution of a strong mineral acid, and washing said acid and solubilized uranium from said mud.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF RECOVERING URANIUM FROM THE LEACHED MUD OF THE CLINKER PROCESS FOR PRODUCING PHOSPHORIC ACID COMPRISING DRYING SAID MUD, GRINDING THE DRIED MATERIAL, ROASTING SAID GROWN AND DRIED METERIAL AT ABOVE ABOUT 1245* F., DIGESTING THE ROASTED METERIAL WITH A DILUTE SOLUTION OF A STRONG MINERAL ACID, AND WASHING THE ACID AND SOLUBILIZRED URANIUM THEREFROM WITH WATER. 